Ab initio vibrational spectroscopy of molecular adsorbates in mordenite

Periodic DFT calculations have been performed to characterize the acidic properties of bridging and terminal OH groups and various defect sites present in zeolites. As a model system mordenite with Si/Al=23 has been used. Binding energies and vibrational frequencies of adsorption complexes of frequently used probe molecules such as CO and NH3 have been calculated. Ammonia is protonated at bridging OH groups while it is H-bonded upon adsorption at terminal silanol groups. A defect with a two-membered silica ring is a weak Lewis site as indicated by very low interaction energies with probe molecules. The Bronsted acidity of a defect with the bridging OH group in the two-membered ring is mid-way between that of bridging OH groups and terminal silanol groups. A three-coordinated surface Al atom is a strong Lewis site.